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Magnetic Permeability of stressed SS
4

Magnetic Permeability of stressed SS

Magnetic Permeability of stressed SS

(OP)
I am designing a deep drawn SS can to be a shield for a magnetic sensor.  We need the relative permeability of the shield to be as close to 1 as possible so as to not interfere with the sensor. Annealed 304 or 316 SS should be on the order of 1.0 to 1.02. What can I expect the permeability value to be for stressed 304 and 316?

RE: Magnetic Permeability of stressed SS

Austenite that has been deformed can have permeability up to 10 or 20. It depends on amount of deformation, temperature at which the deformation occurs, and alloy composition, the higher the total alloying, the less transformation.
Tradionally for the situation you describe, 305 is used. It has been used for deeply drawn TV components which must have very low permeability. Get at least 12% nickel. Sometimes it is sold with as little as 10.5%.
In extreme cases 21-6-9, a nitronic alloy, can be used.

RE: Magnetic Permeability of stressed SS

You can buy 304 with various min Ni also.  The trick is availability.
How much are you going to draw?
I'll try to get you some strain vs perm data.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Magnetic Permeability of stressed SS

4
(OP)
Thank you for the rapid response.
A Rel. Perm of 10 would be unexceptable.  I was hoping to avoid post-draw heat treating because of the extra cost and possibility of added distortion.
The Can is a Cylinder closed at one end with an O.D. at  .750" and about 1.25" depth.  Wall is approx. .02".
We need about 50k pieces annually. Finding a vendor to fabricate this and who also has knowledge of magnetic property inspection has been a problem.
I'll look into 305 and 21-6-9.  Also any charts on perm vs strain would be helpful.

Ron

RE: Magnetic Permeability of stressed SS

When we draw 304L, 316L or 21-6-9 we see no measurable increase in perm.  We are tlaking about standard chemistries.  Our draw reductions are in the range of 20-35% (area reduction).  We still get perms that are not over 1.02.

Just for kicks I measured some tensile samples.  I still don't see numbers over 1.02 at the fracture surface.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Magnetic Permeability of stressed SS

(OP)
I have had experience in the past with 304 becoming magnetic after a deep draw. From your comments, it looks like if the Ni content is on the low end of the range (Ni in 304 is 8 to 10%), the material is more susceptible to becoming magnetic.

305 and 316 has a Ni range of 10 to 13 or 14%.  Is it reasonable to specify 305 or 316 with Ni content greater than 12%? If cost/availability is a problem, what are my alternatives?

Ron

RE: Magnetic Permeability of stressed SS

316 is a killer today.  The surcharges on Mo are huge.  You will end up paying about 30-35% more for 316 than for 304.  Usually the gap is closer to 15%.

I belive that most of the magnetic transfromation that you see on drawn parts is due to the high amount of work on the surfaces.
I see more magnetism in tubing that has been ground than I do in cold drawn product.
You might have better luck (and lower cost) finding some 8.5% or 9% min Ni 304.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Magnetic Permeability of stressed SS

If you want the best value for making this part use 201 with 5% Ni and 7% Mn. Allegheny probably makes it. What counts in surpressing martensite formation is total alloy, not just nickel. Anything you put in except cobalt helps. 201 is the cheapest. I just saw on Allegheny's website that they are promoting it because of nickel's high cost.

RE: Magnetic Permeability of stressed SS

Is it that expensive to do a heat treat after the draw?

RE: Magnetic Permeability of stressed SS

Yes. Plus, you would lose dimensional tolerances.
Very few people in the industry realize that 201 is equivalent to 305 in drawability and magnetic properties. How could anyone sell 305 if this were known? That's one of the reasons we're writing the book.

RE: Magnetic Permeability of stressed SS

(OP)
mcguire:
From the lack of information and often misinformation that I have found, I think this subject would be a valuable addition to your book- especially for the instrumentation/sensor community that needs very low perm. components.  
Specifically need practical advice for non-metallurgist on how to select/specify SS material and fabrication processes to minimize the formation of martensite and obtain low perm. parts.

Ron

RE: Magnetic Permeability of stressed SS

Ron07663
 You'll have it. Can you describe for me a longer list of end uses and technical reasons for wanting low permeability?
I'll have to have an applications chapter on choice of stainless for magnetic properties.
 Anyone else is welcome to offer suggestions. By the way, ASM has formally agreed to publish the book.
 

RE: Magnetic Permeability of stressed SS

congrats mcguire for getting ASMto publish your book. How long will it take to come out? Do you need a proof reader? I can do it if you donot mind.

RE: Magnetic Permeability of stressed SS

Another potentially dumb question.  If this SS can is to be a shield (for electrostatic/electromagnetic fields I assume), wouldn't a non-ferrous metal be a better choice so issues of permiability/residual magnatism would be non-existant?

RE: Magnetic Permeability of stressed SS

(OP)
sreid:
The short answer is that the customer requested SS.
The primary function of the shield is protection from the environment, although it does drain static charge and attenuate EMI.
SS does have some advantages over brass or aluminum - other common deep draw candidates. One of the main advantages is lower eddy current.

RE: Magnetic Permeability of stressed SS

I don't have any references on hand but getting the vibration out of the windmill I recall that the Navy uses a lot of 321 SS down the road at one of their labs that deal with things that need to be non-magnetic.  All my info is for annealed material no mention of cold work.

RE: Magnetic Permeability of stressed SS

(OP)
FYI:
Found this reference with a MAGNETIC PERMEABILITY vs PERCENT  COLD WORK chart for 302, 304, 304L and 305-  http://www.stal.com.cn/pdffile/302304304l305.pdf
It confirms what you all said - 305, with as much Nickel as I can find, seems the way to go. As a plus it is more readily available and lower cost than some other choices.  We will prototype our parts and check the results.

QUESTION -- WHAT SHOULD I SPECIFY ABOUT THE DEEP DRAW PROCESS THAT WOULD REDUCE THE POSSIBILITY OF MARTINSITE?
Draw reduction of 20 -35% was previously mentioned. Anything else?


RE: Magnetic Permeability of stressed SS

If you have a means of drawing it at slightly elevated temperatures, you can prevent the martensite formation. 20F increase in deformation temerature gives the same effect as 1% nickel...pretty good cost trade-off.

The highest nickel 305 commonly available is 12% from Ulbrich. Tell them I sent you. I developed it for them about ten years ago.

RE: Magnetic Permeability of stressed SS

(OP)
mcguire:
To answer your questions on a list of applications for low perm. SS:
Basically any application that requires the properties of steel plus a need to be magnetically transparent.
This list would include magnet sensors to include inductive, hall-effect, GMR technology; Compass direction finders etc. Low perm SS is specified for oil drilling equipments (to not disturb sensitive magnetometers.) Low perm SS is used on various military systems to reduce the magnet signature (example: ship hulls in being evaluated ,etc.) Also low perm SS is specified for components near large magnet fields where it is important not to be attracted to the magnet such as industrial electric welders, superconducting coils in scientific, nuclear and medical (MRI) equipment.

Ron

RE: Magnetic Permeability of stressed SS

"Low perm SS for military systems --".Mcguire this is an important area of application,RON has done well to identify this use. Infact I wanted to discuss with you about this application. The defence of the nation is at stake based on this singular property. However, the work horse alloys are 304 and 304L grades for under water mine applications. I cannot discuss more on this application and magnetic signature on ship's hull.

RE: Magnetic Permeability of stressed SS

Thanks very much for the information.

RE: Magnetic Permeability of stressed SS

If I were doing hulls I would want more austenite stability and pitting resistance than 304 has to make sure no SCC or perforation. What thicknesses are such hulls, if you can say without violating the "secrets act"?

RE: Magnetic Permeability of stressed SS

There have been large scale tests looking at using AL-6XN for ship hulls for mine sweepers.
Resists corrosion
very non-magnetic
strong
high ductility = high damage tolerance

But not at todays prices.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Magnetic Permeability of stressed SS

The Navy did make two SS mine sweepers during WWII but they had cracking problems so they went back to wood.
The Calypso was a ex wooden mine sweeper.

As posted above its an ongoing thing of who is the better at hiding, finding, or blowing up the other, with this work being carried out not far from me.  They don't kill as many fish as they use to as electronics have taken over.

RE: Magnetic Permeability of stressed SS

Does anyone know the permeability of stainless steel at ambient and at cryogenic (4 Kelvin)temperature?

RE: Magnetic Permeability of stressed SS

Vanteen
 This topic is worth a separate thread. It of course depends upon which grade of stainless, but so are actually anti-ferromagnetic near 0 K.

As far as AL6XN for naval vessels, I understand they are getting some major attention because of their superior resistance to explosions ( as distinct from projectile penetration ) and their better acoustic signature than other non-magnetic materials. This alloy beats titanium and HSS armor plate materials.

RE: Magnetic Permeability of stressed SS

(OP)
I thought I would follow up on this thread and basicly close it out.  
We ran sample parts out of our deep draw tool of both 316 SS and 305 SS.  The 305 was selected for Ni content of 12% or greater.
The 316 parts had a measured permeability of 1.15 right out of the tool and 1.0003 after solution anneal.
The 305 parts measured 1.003 right out of the tool.
Our specification required 1.05 MAX.  So, if 316 were used, we would require a post draw anneal.

Thanks again for everyones help with this.

RE: Magnetic Permeability of stressed SS

Thanks for the follow up on a successful resolution of your problem as it doesn't happen very.

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